Chromatography Forum

Hi,

Trying to do acrylonitrile in olive oil by standard addition method. I have problem in detecting 0.1 ug (first spiked standard) acrylonitrile in 5 g oilve oil by headspace GC-FID. My sampling condition are: 5 g oilve oil spiked with 20 ul of acrylonitrile (5 ug/ml) in propylene carbonate/in 20 ml headspace vial/70ºC/1 hour/1 ml injection volume. I saw diferent application for acrylonitrile both in FID and NPD regardless detection limit. Is FID not sensitive enough? I don´t have NPD detector available.

Hi

Would not say it is just the FID. More the headspace technique itself and the method in use in combination with the low levels your looking at.
A total of 0,1ug in the vial is hard to analyse but a rough approximation could be: 50% of 0,1ug acrylonitrile ends up in 10ml gas phase, 1ml is injected with a split ratio of 1:5=> 1 ng (1000pg) on the column. As a comparision you typically can detect about 20pg toluen (that ends up on capillary column) with a FID. Sure there is a difference in the responsfaktor between the two compunds but roughly.

So purely teorethical you should be able to detect it if you can maximise the acrylonitrile concentration in the gasphase and minimise the split ratio.

A few possible ideas to try on a larger concentration to see any enhancement:


1. You refer to an application, did it work and have you done something different?

2. Maximising gasphase concentration.
You are not solving the sample in water so try raising HS oven to 100°C (over bp of acrylonitrile). In water solutions you may use "salting out" technique ie adding a salt like sodiumsulfate to push a polar solvent into gasphase but will likely not help as much here.
Is it possible to increase sample amount?(ie increasing spike)

3. Minimise split ratio. Using an Agilent instrument you typically need to stay on a total flow of 10ml/min (carrier+split).

4. Minimise risk of active surfaces. acrylonitrile is polar so if you use a valve/loop HS make sure that loop/valve/Transfelline is at least 100°C to avoid loss/interactions of analyte.

5. Vial pressurazation. Again mayby depending on what system you use but if you use an Agilent 7694 or the new G18888 you may want to cosider to leave the pressure on but setting time to Zero. At high temperatures the vial pressure should be enough to flood the lines/loop. Pressarusation or to long time may cause a loss/dilution of analyte (leaks etc).

Unsure if there are any specifics you need to pay attention with regarding acrylonitrile have not worked with it, but actetonitrile I have managed to detect at low ug's with a "standard method" (a total of 9ug in 4ml DMF in a 10ml vial, HS oven 70°C, split 1:7).

Seems like you will need to work hard to obtain that sensitivity.

Thanks krickos for your tips,

already working hard on this....
Is the first time I perform this analysis, but I am base on Standard EN 13130-3 (Acrylonitrile in food and food simulants) where they use a NPD detector.
At the moment I am testing HS@100ºC. My Instrument is a Combipal from CTC analytics connect to Varian GC FID with seringe temperature (now) @110ºC.
I let you know next week how is going
thanks again

Is it only the lowest standard in which you cannot detect the acrylonitrile, can you see it in standards with a higher concentration ?

What split ratio are you using, what injection volume, column dimensions, flow rates etc etc ?

Peter

Hi,

Getting better....
With HS@100ºC can now detect 0.5 ug.
Peter, I am using 1:10 split(going down to krickos suggestion); VF624 (30m*0.25mm;1.4um);injection vol:1 ml.Column flow:1ml/min.Injector @200ºC.liner 4 mm single gooseneck no glass wool.

joel

Hi Joel

With a 10:1 split you have a total flow of 11 ml/min plus the septum purge. If your injection speed is faster than this (so if the syringe empties in less than 6 s) you will get a pressure rise in the inlet, and eratic split ratios as sample is lost through the split and septum purge. Are you able to use a column with larger internal diameter - you will then get more of the sample onto the column which will increase peak area.

Peter

Hi,

Getting better....
With HS@100ºC can now detect 0.5 ug.
Peter, I am using 1:10 split(going down to krickos suggestion); VF624 (30m*0.25mm;1.4um);injection vol:1 ml.Column flow:1ml/min.Injector @200ºC.liner 4 mm single gooseneck no glass wool.

joel

Hi

Glad to hear that things are moving your way, in addition to Peters suggestion to increase column ID I am woundering about the liner, it seems "large" for heaspace. Would suggest a low volume one like 200ul to narrow peaks further and increase sensitivity a ~1ml liner tends to cause peak broadning in HS injections.

Cheers

Hi,

Done for now...
HS@110ºC for 2hours and a minimum workable split (1:6) have been suficient to detect 0.1 ug.
Thank you krickos and Peter for your advice. Your suggestions have been very useful for a better understanding of ideal condition of analysis. As my time to finish this one had expired!! I am now happy with the result knowing that I will try your tips as soon as possible to lower even more detection limit.
Joel

Hi Joel

Thanks for the feedback. Just for interest, are you shaking the vials during equilibration ?

Peter

Hi Peter,

Yes, 500 rpm